Field of the Invention
The present invention relates to an image processing apparatus, image processing method, and storage medium for performing a quantization process to form an image on a print medium.
Description of the Related Art
In the case of using a pseudo gradation method to print an image, it is necessary to quantize multi-valued image data, and as a quantization method used for the quantization, an error diffusion method and a dither method are known. In particular, the dither method that compares a preliminarily stored threshold value and a gradation value of multi-valued data with each other to determine dot printing or non-printing has a small processing load as compared with the error diffusion method, and is widely used in many image processing apparatuses. In the case of such a dither method, in particular, dot dispersibility in low gradation range becomes problematic; however, as a threshold value matrix for obtaining preferable dot dispersibility, a threshold value matrix having blue noise characteristics is proposed.
FIGS. 10A to 10C are diagrams for explaining a dither process using a threshold value matrix having blue noise characteristics. FIG. 10A illustrates an example of image data to be inputted into a 10-pixel×10-pixel area. This example shows a state where a gradation value of “36” is inputted into all the pixels. FIG. 10B illustrates a threshold value matrix prepared corresponding to the above 10-pixel×10-pixel area. Each of the pixels is related to any of threshold values of 0 to 254. In the dither method, in the case where a gradation value indicated by multi-valued image data is larger than a threshold value, a corresponding pixel is designated as dot printing “1”. On the other hand, in the case where a gradation value indicated by multi-valued image data is equal to or less than a threshold value, a corresponding pixel is designated as dot non-printing “0”. FIG. 10C illustrates a quantization result based on the dither method. Pixels representing printing “1” are indicated in gray, and pixels representing non-printing “0” are indicated in white. The distribution of printing “1” pixels as seen in FIG. 10C changes depending on threshold value arrangement in the threshold value matrix. By using the threshold value matrix having blue noise characteristics as in FIG. 10B, even in the case where the same pieces of multi-valued data are inputted into a predetermined area as in FIG. 10A, the printing “1” pixels are arranged in a high dispersibility state as in FIG. 10C.
FIGS. 11A and 11B are diagrams illustrating blue noise characteristics and human visual characteristics or a human visual transfer function (VTF) at a visibility distance of 250 mm. In both of the diagrams, the horizontal axis represents a frequency (cycles/mm), indicating lower and higher frequencies toward the left and right of the graph, respectively. On the other hand, the vertical axis represents intensity (power) corresponding to each frequency.
Referring to FIG. 11A, the blue noise characteristics are characterized by, for example, a suppressed low frequency component, a rapid rise, and a flat high frequency component. Hereinafter, a frequency fg corresponding to a peak resulting from the rapid rise is referred to as a principal frequency. On the other hand, the human visual characteristics illustrated in FIG. 11B have high sensitivity in a lower frequency range, but sensitivity in a higher frequency range is low. That is, the lower frequency component is conspicuous, whereas the higher frequency component is inconspicuous. The blue noise characteristics are based on such visual characteristics, and adapted to, in the visual characteristics, hardly have power in the highly sensitive (conspicuous) lower frequency range, but have power in the low sensitive (inconspicuous) higher frequency range. For this reason, when a person visually observes an image subjected to a quantization process using a threshold value matrix having blue noise characteristics, dot deviation or periodicity is unlikely to be perceived, and the image is recognized as a comfortable image.
Note that the principal frequency fg is an average frequency when dispersing a predetermined number of dots as uniformly as possible; however, the principal frequency fg depends on dot density, i.e., gradation.
FIG. 12 is a diagram illustrating the relationship between dot density and a principal frequency fg. In the diagram, the horizontal axis represents a gray level g (i.e., the dot density), and the vertical axis represents the principal frequency fg at each gray level. The gray level g is given on the assumption that a state where dots are placed in all pixels in an image area corresponds to “1”, a state where no dots are placed in all the pixels to “0”, and a state where dots are placed in half of the pixels to “½”. The principal frequency fg in this case can be expressed by Expression 1.
                              f          g                =                  {                                                                                          g                                    ⁢                                                          u                                                                                                                    g                  ≤                                      1                    2                                                                                                                                                                  1                      -                      g                                                        ⁢                                                          u                                                                                                                    g                  >                                      1                    2                                                                                                          (                  Expression          ⁢                                          ⁢          1                )            
In Expression 1, u represents the reciprocal of a pixel spacing. As can be seen from FIG. 12 and Expression 1, the principal frequency fg takes the maximum value of fg=√(½)|u| at a gray level of g=½, i.e., when dots are arranged in 50% of the pixels in the entire pixel area. In addition, as the gray level g separates from ½, the principal frequency fg also gradually shifts toward the lower frequency side.
In the case of performing the dither process, by bringing a peak appearing in frequency characteristics close to a principal frequency, in particularly, in low gradation where visual sensitivity is high and dot density is low, dot distribution can be brought close to a shape having blue noise characteristics. That is, it is possible to achieve a visually preferable state where dots are uniformly arranged without local concentration of dots.
The blue noise characteristics as described above are defined and explained in many literatures such as Robert Ulichney, “Digital Halftoning”, The MIT Press, Cambridge, Mass., London, England. As a method for preparing a threshold value matrix while controlling frequency components, including blue noise characteristics, a void-and-cluster method can be employed. The details of a method for preparing a threshold value matrix using the void-and-cluster method are disclosed in Robert Ulichney, “The void-and-cluster method for dither array generation, Proceedings SPIE, Human Vision, Visual Processing, Digital Displays IV, vol. 1913, pp. 332-343, 1993”.
However, even in the case of using a dither matrix having blue noise characteristics, using multiple color materials makes graininess conspicuous in some cases. Specifically, even though each color material (i.e., single color) results in preferable dispersibility based on the dither matrix, in the case of printing an image using the multiple color materials (i.e., mixed color), dispersibility is lost to make graininess conspicuous in some cases. This seems to be because threshold matrices prepared for the respective color materials do not have any mutual correlation.
U.S. Pat. No. 6,867,884 discloses a dither method for solving such a problem. Specifically, U.S. Pat. No. 6,867,884 discloses a method that prepares one common dither matrix having preferable dispersibility as in FIG. 10B, and performs a quantization process while offsetting mutual threshold values among multiple colors. According to U.S. Pat. No. 6,867,884 as described, dots of different colors are mutually exclusively printed in a high dispersibility state in a low gradation range, and therefore even in a mixed color image, preferable image quality can be achieved.
However, in the case of employing the method disclosed in U.S. Pat. No. 6,867,884 using a threshold value matrix having blue noise characteristics, and focusing on each of color materials, some difference in dispersibility or blue noise characteristics occurs among the color materials. This is because a first color directly using a threshold value in a dither matrix without any offset can enjoy the blue noise characteristics of the dither matrix, but a second or subsequent color using an offset threshold value loses the blue noise characteristics to some extent.
Meanwhile, it is known that in the case of an inkjet printing apparatus using inks (liquids) as color materials, the color chromogenic property, i.e., the conspicuousness of each dot is affected by the order of the color materials to be applied to a print medium.
FIGS. 14A to 14D are schematic diagrams for explaining the order of color materials to be applied to a print medium and states of permeation. The diagrams illustrate a state where a preceding ink 530 containing a first color material 550 and a succeeding ink 540 containing a second color material 560 are applied to substantially the same position of a print medium S in this order. Inside the print medium S, multiple adsorbents 520 are arranged to form an adsorbent layer. In the case where the print medium S is inkjet exclusive paper, the adsorbents 520 are made of alumina, silica, or the like. In the case where the print medium S is plain paper, the fibers of the paper serve as the adsorbents 520.
FIG. 14A illustrates a state immediately before the preceding ink 530 contacts a paper surface 510 of the print medium S. FIG. 14B illustrates a state immediately after the preceding ink 530 has contacted the paper surface 510. The preceding ink 530 having contacted the paper surface 510 spreads in the horizontal direction on the paper surface 510, and then permeates in the depth direction.
FIG. 14C illustrates a state immediately after the succeeding ink 540 has contacted the paper surface 510 with the preceding ink 530 permeating through the paper surface 510. The color material 550 contained in the preceding ink 530 adsorbs to adsorbents 520 near the paper surface 510, and moisture and solvent component other than the color material 550 permeate in the depth direction. On the other hand, the succeeding ink 540 having contacted the paper surface 510 spreads on the paper surface 510, and permeates in the depth direction.
FIG. 14D illustrates a state where the succeeding ink 540 has permeated through the print medium S. At timing when the succeeding ink 540 permeates, adsorbents 520 positioned near the paper surface 510 are mostly covered with the first color material 550. Accordingly, the color material 560 of the succeeding ink 540 cannot adsorb here, and therefore further travels around together with liquid components. Subsequently, the color material 560 passes the adsorbents 520 to which the first color material 550 adsorbs, and adsorbs nearby adsorbents 520. As a result, as compared with the first color material 550, the second color material 560 is unlikely to remain near the paper surface 510, and when observing the print medium S from above, has weak color chromogenic as compared with the first color material 550, making dots inconspicuous.
As described, in an image printed in accordance with an inkjet method, the conspicuousness of each dot depends on the order of inks to be applied to a print medium S. That is, the dispersibility of a preceding ink more significantly affects graininess than the dispersibility of a succeeding ink.